The intrinsic interference technique will be further optimized as a system for the assay of the development of hepatitis A and B viruses in tissue culture of human fibroblast cells. This technique consists of cells infected with noncytopathic viruses (in this case the hepatitis viruses) being refractory to superinfection with New Castle Disease virus (NDV). Replication of NDV is measured by a hemadsorption assay.